Can you possibly not go tomorrow? You don't want to be there and she has said she she will have other people there anyway. She has pretty much made it clear she has other people to support her. So be kind to you and don't put yourself through that stress. (If she gets angry you can always say she said she had others there and you would have to "look for a seat" so you thought it would be too stressful for her to have so many people there).

You don't EVER have to give up on your little one who has always wanted that. You, trailrunner, big and little parts, are awesome. You are always caring, always kind, always considerate, always thoughtful of others above yourself. You bigs and littles have always been good enough.
It is your mother who wasn't.

You tell her you are so sorry she didn't get a good enough mother. tell her it was never ever her fault, not even once. She was born as beautiful and amazing and worthy as every other child on this earth. Tell her it was no fault of her own that she got a mother who couldn't.
Tell her that YOU will be there for her, not matter what, no matter when. That you will always have her back and be her champion and you will try to be the good enough mother that she never had. Tell her you will do your very best to protect her and love her and keep her safe from all harm even if you don't always get it right all the time you are going to try and be the person she always needed but never had.

Don't go. Be kind to you, be kind to little you.

This happened to me a couple weeks ago. It was the mother's birthday. I did actually try to call but didn't get home from work until really late. I got home to a slew of abusive messages from the father on my answer service.
Some people unfortunately expect it rather than earn it.

Yes.

Sending peace to you.